1. Field of the Invention
The present invention relates to a thermoprotector in which the melting point or the softening point of a fusible material is set as the operating temperature.
2. Explanation of Related Art
As a thermoprotector which senses abnormal heating of an electrical or electronic apparatus, and which performs a cut-off operation based on this sense to interrupt the apparatus from a power supply, thereby preventing overheat of the apparatus and occurrence of a fire, a device which operates on the basis of the melting point or softening of a fusible material is known.
Such a thermoprotector has the following basic structure. A movable electrode and a stationary electrode are in contact with each other in a state where elastic distortion energy is stored, the elastic distortion energy is constrained by a fusible member, and, when the fusible member is melted or softened, the elastic distortion energy is released to cause an elastic movable conductor to separate from the stationary electrode.
FIGS. 8 to 10 show examples of such a thermoprotector.
In a thermoprotector shown in FIG. 8, an elastic metal piece 3′ is elastically bent as shown in (8A) of FIG. 8, the both ends of the elastic metal piece 3′ are bonded against a bending reaction force to a pair of stationary electrodes 41′, 42′ by an fusible alloy (solder) 4′ having a predetermined melting point. When the ambient temperature is raised to the melting point of the fusible alloy 4′ and the fusible alloy is melted, elastic bending distortion energy of the elastic metal piece 3′ is released to cancel the joining between one end of the elastic metal piece 3′ and the one stationary electrode 42′ as shown in (8B) of FIG. 8, thereby interrupting the power supply (see Japanese Patent Application Laying-Open No. 7-29481).
In a thermoprotector shown in FIG. 9, as shown in (9A) of FIG. 9, a pellet 2′ having a predetermined melting point, a seat plate 15′, a compression spring 1′, and a seat plate 16′ are sequentially housed in a metal case 14′ to which a lead terminal 13′ is attached at one end, with starting from the one end. Furthermore, a movable electrode 42′ in which the outer circumference is in sliding contact with the inner face of the metal case is housed in the case, a lead pin bushing 17′ is fixed to the other end side of the metal case 14′, and a trip spring 18′ is incorporated between the bushing 17′ and the movable electrode 42′, thereby constituting a conduction path passing the route of the lead terminal 13′→the metal case 14′→the movable electrode 42′→a lead pin 41′. When the ambient temperature is raised to the melting point of the pellet 2′ and the pellet 2′ is melted, compression stress of the compression spring 1′ is released, and the movable electrode 42′ is detached from the tip end of the lead pin 41′ by compression stress of the trip spring 18′ as shown in (9B) of FIG. 9, thereby interrupting the conduction path (see “ELECTRICAL ENGINEERING HANDBOOK” First Edition, The Institute of Electrical Engineers of Japan, Feb. 28, 1988, p. 818).
In a thermoprotector shown in FIG. 10, an elastic movable conductor is elastically flexed by a vertical force due to attachment of a fusible material spacer to be contacted with a stationary electrode as shown in (10A) of FIG. 10, and elastic distortion energy of the elastic movable conductor is released by melting or softening of the fusible material spacer, whereby the elastic movable conductor is detached from the stationary electrode as shown in (10B) of FIG. 10 to interrupt the power supply.
In the thermoprotector shown in FIG. 8, however, the bending reaction force M′ and an n-direction expanding force F′ of the elastic metal piece act on the fusible alloy (solder). Therefore, the stress distribution in the fusible alloy is complicated, and stress acts on a local portion, so that stress concentration inevitably occurs and an operation failure due to creep easily occurs. Since the fusible alloy forms a part of a conduction path, the fusible alloy may generate heat because of an increase of the resistance due to creep of the fusible alloy, thereby causing a possibility that an operation error may be caused by self-heating. Furthermore, an operation error may be caused also by stringing of the molten alloy.
In the thermoprotector shown in FIG. 9, the pellet can be uniformly compressed by pressure equalization of the seat plates, but the structure is complicated. Therefore, the thermoprotector is inevitably disadvantageous in miniaturization and cost.
In the thermoprotector shown in FIG. 10, the elastic movable conductor is caused by the vertical force to be contacted with the stationary electrode, and the contact is cancelled in the vertical direction. Therefore, a space for installing the fusible material spacer must be disposed in the vertical direction, and therefore this stricture is disadvantageous in low-profiling of a thermoprotector.